Recent developments in fuel cell (FC) technology have resulted in compact, light-weight devices having the ability to continuously produce electrical power via electrochemical reactions involving an oxidant (usually, air) and a fuel (most preferably, hydrogen). These devices can potentially replace conventional batteries and are particularly useful for portable electronic systems, space and military (e.g. “soldier power”) applications, transportation and other systems. However, a compact FC would require a similarly compact and light-weight source of hydrogen. Unfortunately, conventional means of storing hydrogen, including compressed gaseous or liquid hydrogen, as well as, hydrogen cryogenically adsorbed on activated carbons do not lend themselves to use in portable devices. Similarly, the advanced methods of hydrogen storage, such as metal hydrides, catalytically enhanced metal hydrides, carbon nanotubes, also result in relatively low hydrogen storage capacities (below 6 w. %).
To overcome hydrogen storage problems, there have been attempts to produce hydrogen “on demand”, i.e. generate hydrogen as needed. In this respect, hydrogen generating systems based on the reactions of different metal hydrides with water are most developed. For example, U.S. Pat. No. 3,459,510 to Litz et al. describe a hydrogen generator which produces hydrogen via the reaction of metal hydride (preferably, sodium borohydride, NaBH4) with water. The reaction was enhanced by the presence of a metal catalyst (preferably, Raney nickel). Recently, Amendola et al. described a catalytic system for generating hydrogen from aqueous borohydride solutions using a ruthenium catalyst (S. Amendola, M. Binder, M. Kelly, P. Petillo, S. Sharp-Goldman, Advances in Hydrogen Energy, Ed. C. Padro and F. Lau, Kluwer Academic/Plenum Publ., NY, 2000). U.S. Pat. No. 4,155,712 to Taschek and U.S. Pat. No. 5,593,640 to Long et al. deal with portable hydrogen generators based on the hydrolysis and thermal decomposition of metal hydrides (e.g. CaH2, LiAlH4). The common disadvantages of these systems based on hydrolysis of metal hydrides is the necessity to carry water and, most importantly, the requisite use of expensive metal hydrides which are irreversibly hydrolyzed into metal hydroxides during hydrogen production. Thus, these systems would require handling of metal hydroxide slurries, which would be very difficult, energy intensive and costly to convert back to original hydride form.
To overcome above problems, a variety of hydrocarbons and other hydrogen containing substances (e.g. alcohols) have been widely used as a source of hydrogen. Since hydrocarbon fuels can be obtained from different sources (petroleum or natural gas), they are abundant, cheap and readily available. There has been a significant progress in the area of development of hydrocarbon reformers (or processors) for producing hydrogen or hydrogen-rich gas for fuel cell applications. For example, U.S. Pat. No. 5,932,181 to Kim et al. describes a hydrogen generator which is able to produce high purity hydrogen from natural gas and water. The generator comprises a desulfurization reactor, a steam reformer, a CO conversion (or water gas shift, WGS) reactor and pressure swing adsorption (PSA) unit. Thus, the process includes the following stages:a) steam reforming: CH4+H2O6CO+3H2  (1)b) CO conversion: CO+H2O6CO2+H2  (2)c) H2 purification (CO2 removal) via PSAThe system also requires a large number of heat exchangers, compressors, valves, etc. The steam reforming (SR) process requires a source of water which adds to the weight of the system. This would result in a bulky system potentially difficult to miniaturize. A similar hydrogen generating apparatus is described in U.S. Pat. No. 5,110,559 to Kondo et al. The apparatus consists of a hydrocarbon and air supply systems, a steam generator, a steam reformer, a CO shift converter and gas separation system (PSA). U.S. Pat. No. 5,470,360 to Sederquist discloses an advanced steam reformer with improved distribution of heat within the apparatus. The reformer is applicable to a fuel cell power plant. All these apparatuses are complex and bulky and suffer from the same disadvantages as the previous one. An improved method for producing hydrogen from sulfurous hydrocarbon fuels in the fuel cell electricity generation process is described in U.S. Pat. No. 5,284,717 to Yamase et al. The method is characterized by cracking and desulfurizing petroleum fuels (e.g. kerosene) followed by steam reforming of the desulfurized product. The process is multi-stage, complex and would require an additional source of water. Another multi-stage process for production of hydrogen and energy is described by Engler et al. in U.S. Pat. No. 5,888,470. The process is based on partial oxidation of hydrocarbon fuel and includes a reformer, CO-converter and two gas separation units (a membrane and PSA). Partial oxidation (PO) of natural gas (catalytic and non-catalytic) can be described by the following equation:CH4+½O2→CO+2H2  (3)This reaction is followed by CO conversion (or WGS) and gas separation (PSA).
A compact hydrogen generator is disclosed in U.S. Pat. No. 4,737,161 to Szydlowski. The apparatus catalytically reforms a hydrocarbon fuel to a hydrogen rich gas for fueling a fuel cell stack. The device features a cylindrical housing with an axial burner and a helical catalyst tube outside of the burner and inside of the housing. U.S. Pat. No. 5,780,179 to Okamoto describes a steam reformer-fuel cell system, where hydrogen is humidified by the recycled water. U.S. Pat. No. 5,938,800 to Verrill, et al. and U.S. Pat. No. 5,897,970 to Isomura et al. describe multi-fuel compact reformers for fuel cell applications. Both patents feature a steam reformer coupled with a hydrogen separating membrane (e.g. Pd-based membrane). The use of selective hydrogen membrane allows to avoid bulky PSA system for the separation of gases and can potentially result in smaller units. However, the apparatuses are complex and require to carry a significant amount of water (e.g. in the first patent, 2-4 moles of steam per mole of carbon in the fuel) (although, some amount of water from the exhaust of FC could be recycled to the reactor). U.S. Pat. No. 5,997,594 to Edlund et al. discloses a steam reformer with internal hydrogen purification as a source of high purity hydrogen for a polymer electrolyte fuel cell (PEFC). The apparatus includes a steam reformer with internal bulk hydrogen purification and polishing, an integrated combustion method utilizing waste gas to heat the reformer, an efficient integration of heat transfer, resulting in a compact design of the unit. Hydrogen is purified using a thin Pd alloy membrane. A steam-hydrocarbon reformer with a catalytic membrane is disclosed in U.S. Pat. No. 5,229,102 to Minet et al. The use of ceramic membrane permeable to hydrogen and carrying catalytically active metallic substance allowed to significantly reduce the maximum temperature of the process and simplify the design. A steam/methane weight ratio of 3/1 to 5/1 was required to operate the reactor.
A number of patents (e.g. U.S. Pat. Nos. 5,366,819; 5,763,114; 5,858,314; 5,641,585; 5,601,937) deal with a hydrogen generator integrated with a solid oxide fuel cell (SOFC). For example, U.S. Pat. No. 5,366,819 to Hartvigsen et al. discloses a reformer integrated with SOFC. A thermally integrated steam reformer is located inside the stack furnace housing stacks of SOFC. De-sulfurized natural gas as a feedstock was reformed with a steam over Ni- or Ru-based catalysts. Similarly, U.S. Pat. No. 5,641,585 to Lessing et al. describes a miniature ceramic SOFC with a built-in hydrocarbon reformer using Ni-catalyst. Light hydrocarbons (e.g. propane and butane) are preferred feedstocks for this apparatus. There are several disadvantages common to all these integrated hydrogen generator-SOFC systems. First of all, very high operating temperatures at which SOFC operate (approx. 1000° C.) make them inconvenient for incorporation into small hand-held or other portable devices. Secondly, a preferred feedstock for these devices are desulfurized light hydrocarbon fuels (methane, propane, butane, naphta). The use of heavier hydrocarbon feedstocks (gasoline, diesel fuel, etc.) might promote excessive hydrocarbon cracking reactions causing carbon deposits on the catalytic surfaces, and clogging interstices within the reactor bed. Thirdly, a feedstock should be carefully desulfurized to avoid deactivation of sulfur-sensitive catalysts (Ni or Ru). Desulfurization unit will add to the weight of the apparatus. Fourthly, a significant amount of water might be necessary to drive the reformation reaction and to prevent from coke deposits inside the integrated system. Lastly, they may not be suitable for military applications, since they have an acoustic, thermal, and chemical (CO2) “signatures”.
Development of small fuel reformers based on thermocatalytic decomposition (TCD) of hydrocarbons for fuel cell applications has been active area of research. The concept is based on a single-step decomposition (cracking, pyrolysis) of hydrocarbons in air/water-free environment. For example, Calahan has developed a fuel reformer to catalytically convert different hydrocarbon fuels to hydrogen which was fed to a 1.5 kW fuel cell (Callahan, M. Proc. 26th Power Sources Symp., Red Bank, N.J., 1974; p. 181). A stream of gaseous fuel entered one of two reactor beds, where hydrocarbon decomposition to hydrogen took place at 870-980° C. and carbon was deposited on the Ni-catalyst. Simultaneously, air entered the second reactor where the catalyst regeneration by burning coke off the catalyst surface occurred. The streams of fuel and air to the reactors then were reversed for another cycle of decomposition-regeneration. The reported fuel processor did not require WGS and gas separation stages, which was a significant advantage. However, hydrogen was contaminated by carbon oxides which required additional purification step. It was reported most recently on the development of similar hydrocarbon fuel processors based on the catalytic decomposition of natural gas and propane for fuel cell applications (e.g. Pourier, M; Sapundzhiev, C. Intern. Journal of Hydrogen Energy, 1997, v. 22, p. 429-433). Hydrocarbon feedstock is thermocatalytically decomposed in the reactor at elevated temperatures (900-1000° C.) with the production of hydrogen-rich gaseous mixtures and carbon (coke) deposited on the metal (Pd) catalyst surface. After completion of hydrocarbon decomposition stage, air is introduced into the system to burn coke off the catalyst surface. At the heart of the concept is a two-reactor (packed-bed type) system where one reactor is used for hydrocarbon decomposition while the other one is being regenerated. Thus, the whole process runs in a non-steady state regime using complex gas distribution and reagent delivery system. Due to cyclic nature of the process, hydrogen is contaminated with carbon oxides which requires an additional stage of purification using methanator. Furthermore, the above hydrogen generators utilize a number of moving parts, and produce large volumes of CO2 emissions.
Several patents are concerned with the use of carbon-based catalysts for decomposition of hydrocarbons into hydrogen and carbon. For example, U.S. Pat. No. 4,056,602 to Matovich deals with the high temperature apparatus to carry out thermal reactions, including decomposition of hydrocarbons, utilizing fluid wall reactors. Thermal decomposition of methane was conducted at 1260-1871° C. utilizing carbon black particles as adsorbents of high flux radiation energy, and initiators of the pyrolytic dissociation of methane. However this apparatus operates at excessively high temperatures and would be very difficult to miniaturize due to its complexity. In U.S. Pat. No. 5,650,132, Murata et al. claim the process for producing hydrogen from methane and other hydrocarbons by contacting them with fine particles of a carbonaceous material obtained by arc discharge between carbon electrodes and having an external surface area of at least 1 m2/g. Carbonaceous materials also included soot obtained by thermal decomposition of different organic compounds (or combustion of fuels), carbon nanotubes, activated charcoal, fullerenes C60 or C70, and finely divided diamond. The optimal conditions for methane conversion included: methane dilution with an inert gas (preferable methane concentration: 0.8-5% by volume), the temperature range of 400-1,200° C. and residence times 0.1-50 s. Increase in methane concentration in feedstock from 1.8 to 8 v. % resulted in a drastic drop in methane conversion from 64.6 to 9.7% (at 950° C.). Thus, diluted methane feedstock would result in a diluted hydrogen stream which could significantly reduce the efficiency of FC. Also, it was suggested that oxidizing gases like H2O or CO2 be added to the pyrolyzing zone to improve the catalyst life. However, this would inevitably contaminate hydrogen with carbon oxides and require additional purification step. An International Application No. WO 00/21878 to Arild discloses a method and a device for production of hydrogen and carbon by pyrolysis of natural gas and other organic gases for fuel cell applications. According to this invention, pyrolysis of hydrocarbon feedstock takes place in the reaction chamber with an increasing temperature gradient in the direction of flow from 300 to 2000° C. The reaction chamber contains finely distributed carbon dust which acts as a catalyst. Hydrogen is purified by a membrane filter. The growing carbon particles are trapped by means of a mechanical system (e.g. a centrifuge) in the lower part of the reaction chamber, from where a controlled fraction of carbon particles is recycled, crushed and injected to the upper part of the chamber. There are several aspects of this invention that would make it very difficult to produce a compact device. Most difficult is the transporting of a controllable amount of solid carbon from the bottom of the reaction chamber through a crushing device to the upper part of the chamber. Secondly, it would be difficult to control the temperature gradient in the reaction chamber if a heat source other than electric coil will be used to drive the endothermic process. Thirdly, the temperature in the upper part of the reaction chamber is 2000° C., which could potentially affect the operation of a closely located membrane. Lastly, the device has many moving parts (e.g. a centrifuge, a carbon crusher, a carbon transporter) which could make it bulky, noisy, and prone to malfunction.
Thus, there is a need for a simple, efficient, compact, cost-effective hydrogen generator which can be coupled to or integrated with a fuel cell to produce a portable source of electrical power.